The present invention relates to method and apparatus for molding articles of structural foam. In structural foam molding, a molded article with a solid unfoamed skin and a cellular inner core has long been known to be desirable. Conventional structural foam molded parts are sometimes objected to on the grounds that they have surface imperfections, such as swirls, blisters, pin holes, pot marks, streaks and the like. These surface irregularities are produced by the expanding foam gas bubbles, as they move across the cold mold surface during filling of the mold, being permanently solidified by the colder mold surface.
In conventional structural foam molding processes in commercial use, the gas bubbles start to form when the resin enters the mold cavity, the bubble growth rate being a function of the pressure drop between the pre-pressurized resin and the mold cavity pressure which is usually at atmospheric pressure. U.S. Pat. Nos. 3,268,636 and 3,436,446 illustrate conventional processes.
The blowing agents used in the conventional structural foam molding processes can be classified as either physically or chemically generated, by which is meant the blowing agent is either a gas that is mixed with the resin in the plasticator or the blowing agent is a solid material which decomposes in the plasticator in response to heating of the solid blowing agent. Both physical and chemical blowing agents are introduced and mixed in the extruder and generally dissolve into the molten resin at the pressures and temperatures found in the resin molding system. Various prior art patents disclose processes utilizing these blowing agents which are intended to overcome the problem of surface imperfections on the molded parts.
For example, U.S. Pat. No. 3,988,403 discloses a system in which the mold surfaces are heated prior to introduction of the molding resin into the mold cavity in order to soften and "smooth out" surface imperfections. Such a system is objectionable because thermal cycling of the mold from hot to cold has many economic disadvantages such as increased energy consumption, increased cycle time and increased mold costs.
U.S. Pat. No. 4,255,367 attempts to solve the surface imperfection problem by selectively adding foaming agents to the melt stream during injection. In this way, a solid unfoamed material can be first injected into the mold to form the skin and then a foaming agent is introduced in the melt stream and injected into the core to form the cellular core structure. Proper sequencing of the injection mold fill cycle becomes critical and residual foam material must be purged from the system before the solid injection phase of the next cycle can begin. Since the foaming agent injection point is upstream of the manifold system and the injection nozzles considerable volume can exist to store unwanted material between shots. This residual foam material may be inadvertently injected into the mold causing imperfections in the surface of the part. Also, in some cases, the foam core material breaks through the mold surface. The mixing of core and skin materials adversely affects the surface of the molded article.
U.S. Pat. No. 4,255,368 discloses a method for producing a molded part with a foamed core and a non-foamed exterior shell by using a physical foaming agent such as nitrogen. The foaming agent is introduced into the polymer melt directly in the extruder prior to mold filling. A pressurized mold is used to retard the growth of foam bubbles during injection, the foam bubbles forming only after the mold has been completely filled. This process is objectionable because of the necessity to pressurize the mold with the attendant requirement for sealing valves and gaskets.
U.S. Pat. No. 4,247,515 discloses a system capable of forming articles with solid skins but not foamed cores. The process disclosed in this patent produces parts with solid color and smooth surfaces but there is no provision for a foamed core.
U.S. Pat. No. 4,129,635 shows a process for producing an article with a solid skin and a foamed interior. A foaming agent is introduced into the resin in the extruder and the mixture is injected into a pressurized mold to retard bubble expansion. This structure is objectionable because of the requirement for special molds designed for pre-pressurization and also capable of withstanding very high mold cavity pressures.
The object of this invention, therefore, is to provide an improved method and apparatus for forming plastic parts with exterior solid surfaces and foamed cores which overcomes the above objections.